<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        /* 
            一个方形，
            前面的是个三角形，定位上去
        */
        .box {
            width: 120px;
            height: 80px;
            background: red;
            position: relative;
            -moz-border-radius: 10px;
            -webkit-border-radius: 10px;
            border-radius: 10px;
        }

        .box:before {
            content: "";
            position: absolute;
            right: 100%;
            top: 26px;
            width: 0;
            height: 0;
            border-top: 13px solid transparent;
            border-right: 26px solid red;
            border-bottom: 13px solid transparent;
        }
    </style>
</head>

<body>
    <div class="box"></div>
</body>

</html>